In the formation of silver halide emulsions for use in photographic materials, a desirable characteristic is to have photographic materials that are capable of being rapidly processed. It is known that silver halide emulsions having high levels of chloride content (greater than 90 mole % chloride) are especially useful in achieving rapid processability due to their higher solubility compared to emulsions having greater than some minimum levels of other halides (for example, emulsions having greater than 10 mole % bromide or more than 3-5 mole % iodide or both).
Silver halide emulsions including those high in chloride content are also known to require some form of chemical sensitization in order to increase their photographic efficiency. Chemical sensitization of an emulsion involves the addition of one or more chemical sensitizing agents where the sensitizing agent is capable of undergoing a chemical reaction on the silver halide grain surface during the application of thermal energy for some time period. The chemical sensitization involving adding chemical sensitizer to an emulsion and heating is often referred to as chemical digestion of the silver halide emulsion. Emulsions, especially high chloride content emulsions, also require spectral sensitization involving the addition of surface adsorbing sensitizing dyes to the emulsion grains which make the grains sensitive to specific wavelengths of light. In addition, silver halide emulsion grains are often treated with other photographically useful chemical compounds such as salts of other halides which can cause surface conversion of the host emulsion grains to a mixed halide composition. Other known photographically useful compounds which can be added are antifoggants, stabilizers, metal dopants, silver halide solvents, ripeners, supersensitizers, coating aids, and surfactants. These photographically useful compounds can be added prior to, during, or after the chemical sensitization or the spectral sensitization steps.
It is also known that silver halide emulsions, especially those high in chloride content, can be caused to fog or to exhibit high D.sub.min density as a result of the application of heat during the chemical sensitization (digestion) step. The onset of this undesirable fog or high D.sub.min is often the limiting factor in the performance of photographic materials composed of silver halide emulsions, and there is a continuing need to improve the speed/fog performance of these sensitized silver halide emulsions, especially emulsions containing high levels of chloride. The terms "speed" and photographic "sensitivity" are used interchangeably herein.